1. Field of the Invention
This invention relates to a novel thermal medium oil and more particularly to a novel thermal medium oil having excellent oxidation stability and comprising at least one monosubstituted naphthalene derivative having a specific structure.
2. Prior Art
The recent technical progress in chemical industries is remarkable. In the heating system, an indirect heating system using an oil or the like as a thermal medium has been substituted for a conventional direct heating system and is now widely used in all the fields of industries such as fiber, paper making, foodstuff, architectural and chemical industries.
A heat or thermal medium oil has most generally been used as the thermal medium for the indirect heating system and is usually required to be such that:
(1) It is excellent in oxidation stability, PA1 (2) It has a low vapor pressure and a high flash point, PA1 (4) It has no toxicity and no odor, and PA1 (5) It has good heating efficiency. PA1 (1) the hydrocarbon radical is a monosubstituted naphthalene, PA1 (2) R.sub.1, R.sub.2 and R.sub.3 of the hydrocarbon radical represented by the general formula ##STR2## may be identical with, or different from, each other and are each an alkyl, phenyl or alkylphenyl group having 1 to 21 carbon atoms with the proviso that the total of carbon atoms of R.sub.1, R.sub.2 and R.sub.3 is 4 to 23, and PA1 (3) the hydrocarbon radical is a tertiary one, i.e., it is attached directly to the naphthalene ring via a tertiary carbon atom. The above three requirements must be met for the purpose of this invention. Naphthalene derivatives which fail to meet even one of said three requirements are undesirable since they are inferior to those used in this invention in the respects of oxidation stability and other physical properties necessary as thermal media.
(3) It has satisfactory fluidity even at low temperatures,
At the present, there are widely used not only a highly refined mineral oil incorporated with an antioxidant, but also a phenyl ether, a polyphenyl, an arylalkane and an alkylnaphthalene having methyl, ethyl, propyl or the like group, and the like.
Of these known thermal medium oils, the alkylnaphthalene-based ones are non-toxic and have a low viscosity, low melting point and high boiling point which are preferable requirements for thermal medium oils, however, they are still not satisfactory in oxidation stability.
The present inventors noted the alkylnaphthalene-based thermal medium oils and made intensive studies in attempts to develop or obtain alkylnaphthalene-based thermal medium oils having higher oxidation stability and, as the result of their studies, they found that thermal medium oils comprising at least one monosubstituted naphthalene derivative have remarkably high oxidation stability as compared with the known alkylnaphthalene-based ones. This invention is based on this finding or discovery.